A hard-disk drive (HDD) is a non-volatile storage device that is housed in a protective enclosure and stores digitally encoded data on one or more circular disks having magnetic surfaces. When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head that is positioned over a specific location of a disk by an actuator. A read/write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. Write heads make use of the electricity flowing through a coil, which produces a magnetic field. Electrical pulses are sent to the write head, with different patterns of positive and negative currents. The current in the coil of the write head induces a magnetic field across the gap between the head and the magnetic disk, which in turn magnetizes a small area on the recording medium.
Increasing areal density (a measure of the quantity of information bits that can be stored on a given area of disk surface) is one of the ever-present goals of hard disk drive design evolution. In turn, as recording tracks in HDDs become narrower and narrower and bits are recorded smaller and smaller, there is a need for more accurate and sustainable head positioning, sometimes referred to as “track following”. One of the factors impairing precise track following is repeatable runout (RRO), which generally refers to the deviation of the head from the theoretical perfect circle of the track. Among other factors, RRO is affected by temperature changes within an HDD because the interlocked components such as the disk spindle, the disk clamp, and the disk itself typically have different thermal expansion attributes.
Modern HDDs include servo systems that read and interpret servo bursts on the disk tracks, which contain special data that the head reads and the servo system firmware interprets to determine to which track the head is closest and how far off track center the head is. While servo systems can compensate for RRO, the HDD performance is detrimentally impacted in doing so due, for example, to some degree of latency involved with any such compensation scheme. Thus, the manner in which RRO is handled within HDDs is an important factor in improving the performance of HDDs.
Any approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.